The present invention has as its object a screw with recirculated satellite rollers used in mechanical engineering for imparting to any object an extremely precise movement of translation of the order of a micron.
Numerous situations are encountered in industry in which a piece is required to be displaced along a straight line over a very precise distance. This applies, for example, to an optical measuring instrument for a lens in which the position is adjusted in relation to a light ray. Another example is found in rolling mills where the space between two rollers measuring about 100 kg each is adjusted to an accuracy of 10 microns.
The problem in these situations is to convert a displacement which is visible on a human scale, i.e. of the order of a centimeter into an infinitesimal movement. One of the numerous methods of reduction available for this purpose consists in the rotation of a screw inside a fixed nut. For a screw with an average pitch of 1 mm, one complete turn of the screw corresponds to a longitudinal displacement of the screw by 1 mm and one hundredth of a turn corresponds to a displacement of 10 microns. It is at present easy to execute a rotation of 1/100 of a turn, for example with the aid of a handwheel 1 m in circumference connected to the screw or by means of an electric stepping motor.
Now the sliding friction which occurs particularly between a screw and its nut has the characteristic of abruptly decreasing at the beginning of a movement. In other words, the initial resistance to movement of a stationary screw is considerably greater than the resistance when the screw has already begun to rotate. This phenomenon hinders very small displacements since it is impossible to reduce immediately the couple initially applied to the screw for setting it in motion.
To overcome this disadvantage, a set of rollers with circular threads are placed round the screw between the screw and the nut to replace the sliding friction by a constant rolling friction. In order to reduce this rolling friction as much as possible, the whole arrangement is manufactured from a hard material such as steel and the cross-section of the threads of both the screw and the nut are set at 90.degree. along straight sides so that there is only a single point of contact between each circular thread and the respective threads of the screw and the nut. A fresh problem then arises from the fact that these rollers, which turn with the screw, are also displaced in the longitudinal direction and are therefore liable to escape from the nut. It is therefore necessary to provide a mechanism to recirculate the rollers, that is to say to return them regularly to the center of the nut.
This recirculation is normally achieved by means of a longitudinal groove formed in the thread of the nut and two cams situated respectively at the entrance and exit of the latter. If, therefore, a roller moves into position in front of this groove in the course of its rotation around the screw, it is pushed away from the threads of the screw by one of the two cams which direct it towards the interior of the nut by a distance equivalent to one or two screw pitches according to the amount of projection of the cam. This roller is then pushed laterally by the roller cage to return to the correct threads where it resumes its course.
This system has, however, numerous disadvantages in operation. When the roller abuts against the cam, a slight shock is produced. When the system is used intensively, for example when there is a continuous reciprocating movement at a speed of 300 revs/min, the sequence of these shocks generates vibrations in the machine as a whole as well as producing unpleasant noise. Depending on the design of the machine, these vibrations are liable to amplify and result in premature wear which jeopardizes the general reliability of the machine. Moreover, the wear on the cam produces iron filings which are liable to infiltrate the screw threads and thereby destroy the precision of displacement of the screw.
Another disadvantage arises from the necessity of the roller to be displaced by the cam by a distance strictly equal to the pitch of the screw because otherwise a so-called "hard point" would be created, that is to say a significant amount of jamming at the moment when the roller re-engages with the threads of the screw and the nut. Since the cam and the roller are manufactured separately, this requires precise adjustment of the height of the shoulder of the cam and of the surface of contact of the roller in relation to its grooves. Such stringent conditions would result in a considerable proportion of the pieces manufactured being lost as rejects and would require very careful and delicate assembly which would increase the final cost of the screw.